Recently, electric motors tend to be driven by an inverter of pulse width modulation (PWM) system. In the case of such PWM system inverter driving, the neutral point potential of the winding is not zero, but a common mode voltage is generated. By this common mode voltage, a potential difference occurs between the inner ring and the outer ring of the bearing (which is called an axial voltage hereinafter). The axial voltage contains high frequency components due to switching, and when the axial voltage reaches a dielectric breakdown voltage of the oil film in the bearing, a high frequency current flows into the bearing, and an electrolytic corrosion occurs in the inside of the bearing. When the electrolytic corrosion is progressed, a wavy abrasion phenomenon may occur inside of the inner ring or the outer ring of the bearing, which may lead to occurrence of abnormal sound, and it is one of the principal causes of troubles in an electric motor (see, for example, patent document 1).
A particularly likely condition to cause an electrolytic corrosion is known to occur where the motor applied voltage is high (for example, in commercial power source 240 V district), the temperature is relatively low, and the motor is operated for a long time in a small variation state of rotating speed.
Conventionally, to prevent the electrolytic corrosion, the following measures have been proposed.
(1) To set the inner ring and the outer ring of the bearing in conductive state.
(2) To set the inner ring and the outer ring of the bearing in insulated state.
(3) To lower the axial voltage.
A specific method of (1) is, for example, to use a conductive lubricant for the bearing. However, the conductive lubricant is worsened in conductivity in the course of time, or is inferior in sliding reliability. Alternatively, a brush may be installed on a rotation shaft to set in conductive state, but this method requires a brush abrasive powder or a space.
A specific method of (2) is, for example, to change iron balls in the bearing to ceramic balls. This method is very effective for preventing electrolytic corrosion, but it is very costly and cannot be applied in general electric motors.
A specific method of (3) is, for example, to install a voltage dividing circuit for lowering the axial voltage in parallel to the axial voltage portion. More specifically, a metal disk (conductor) is provided in the shaft of the electric motor, and this disk is brought closer to a bracket to form an electrostatic capacity. A method of lowering the axial voltage by such configuration is known (see, for example, patent document 2).
However, in the configuration as disclosed in patent document 2, it is necessary to provide the shaft with a disk, and the number of components and the number of manufacturing processes are increased. Further, to increase the electrostatic capacity, it is necessary to increase the surface area of, for example, the disk, and a large structure is needed, and it is not suited to downsizing of the electric motor. Therefore, in consideration of application in small-sized electric motor or mass production, it was very difficult to employ the technique as disclosed in patent document 2.    Patent document 1: Japanese Patent Application Unexamined Publication No. 2007-159302    Patent document 2: Japanese Patent Application Unexamined Publication No. 2000-152564